In general, a micro-resonator sensor detects the characteristics of a measurement-subject material by detecting the intensity of light at an output terminal of a waveguide which corresponds to a change in an effective refractive index of a ring resonator, which is installed separately from the waveguide, when light proceeding through the waveguide including an input terminal and the output terminal is coupled to the ring resonator.
FIG. 1 illustrates the related art micro-ring resonator sensor.
With reference to FIG. 1, the related art micro-ring resonator sensor includes a main waveguide 110 and a ring resonator 120. The main waveguide 110 is formed as an optical fiber or an optical waveguide (or light waveguide), and both ends of the main waveguide 110 serve as an input terminal to which optical signal is inputted and an output terminal from which the optical signal is outputted, respectively. The ring resonator 120 is an annular optical fiber or an optical waveguide with a certain radius (R), including an opening 122 whose surface is interface-treated so that light proceeding through the optical fiber or the optical waveguide constituting the ring resonator 120 can effectively react to a liquid or gas, a measurement-subject material. Such opening 122 is formed at an upper surface or a side surface of the optical fiber or optical waveguide constituting the ring resonator 120. A optical transmission mode that can be accommodated by the micro-ring resonator sensor is determined depending on where the opening 122 is formed. Thus, if the opening 122 is formed at both on the upper surface and on the side surface of the ring resonator 120, the micro-ring resonator sensor can receive both optical signals of a TM mode and a TE mode. The main waveguide 110 and the ring resonator 120 are separately disposed on a single dielectric substrate to constitute the ring resonator sensor.
In the related art micro-ring resonator sensor as shown in FIG. 1, an optical signal inputted through the input terminal of the main waveguide 110 proceeds along the main waveguide 110 and is then coupled to the ring resonator 120 disposed to be separated from the main waveguide 110 according to resonance conditions of the ring resonator 120. The light incidented to the ring resonator 120 is reacted to a liquid or gaseous bio-material, a measurement-subject material, on the interface-treated surface of the opening 122 formed at the ring resonator 120, and accordingly, an effective refractive index of the ring resonator 120 changes. The change in the effective refractive index of the ring resonator 120 triggers a change in conditions for optical coupling from the main waveguide 110 to the ring resonator 120. Namely, the effective refractive index of the ring resonator 120 changes according to the density of the material reacting on the upper surface and on the side surface of the ring resonator 120, and accordingly, the amount of outputted light through the output terminal of the main waveguide 110 varies, thus detecting the characteristics of the material. Configuration of a biotransducer by introducing a biological element to the opening 122 of the ring resonator 120 makes it possible to fabricate a bio-sensor using the ring resonator.
If the resonance conditions are met, optical coupling from the main waveguide 110 to the ring resonator 120 occurs, and if threshold coupling conditions are met, the optical signal is not outputted to the output terminal of the main waveguide 110. The intensity of the optical signal coupled from the main waveguide 110 to the ring resonator 120 is maximized at a point of time when the threshold coupling conditions are met.
FIG. 2 is a graph of characteristic curved line of an outputted light versus wavelengths of incident light according to the resonance conditions of the ring resonator 120. With reference to FIG. 2, when a threshold coupling occurs under the resonance conditions of the ring resonator 120, there is no output from the output terminal of the main waveguide 110 at a minimum wavelength, and the minimum wavelength is shifted according to interaction between bio-molecules. Namely, the wavelength of a optical signal at which no output is generated from the output terminal of the main waveguide 110 varies according to a variation of the effective refractive index of the ring resonator 120 by the measurement-subject material brought into contact with the opening 122 of the ring resonator 120. In FIG. 2, it is noted that each time the effective refractive index of the ring resonator 120 increases by 1×10−4, the minimum wavelength at which there is no output from the output terminal of the main waveguide 110 is increased uniformly. Thus, the ring resonator sensor can detect the characteristic of the measurement-subject material by measuring a response signal with respect to the strength (intensity) and wavelength of the optical signal output through the output terminal of the main waveguide 110.
The output of the ring resonator sensor is very sensitive to a change in the dielectric constant of a medium occurring when the medium comes in contact with the opening 122 formed at the ring resonator 120. Namely, as the medium flows (moves) through the opening 122 of the ring resonance sensor, the dielectric constant of the medium changes, and accordingly, the effective refractive index of the ring resonator 120 is also changed. Such change in the effective refractive index of the ring resonator 120 causes the resonance conditions to be changed, making the output wavelength shifted. Thus, the ring resonator sensor detects the characteristics of the measurement-subject material by detecting the density of the measurement-subject material by the effective refractive index of the ring resonator 120 calculated based on the strength and the phase of the optical signal measured at the output terminal of the main waveguide 110.
However, the related art ring resonator sensor is advantageous in that the characteristics of the measurement-subject material can be measured through the simple structure, but it has a limitation in terms of reducing the size of the sensor. Namely, for the related art ring resonator sensor including the resonator with an optical waveguide in the circular loop form, in order to reduce the radius of the ring resonator without an excessive radiation loss, the periphery of the optical waveguide constituting the ring resonator needs to be deeply etched. Deeply etching the periphery of the optical waveguide can enhance a side optical confinement effect of the optical waveguide, but increases a optical propagation loss due to a sidewall roughness. In addition, if the optical waveguide itself constituting the ring resonator is made of an intrinsic material, etching through the intrinsic material causes a problem due to an excessive surface recombination. In addition, such ring resonator increases a radiation loss, resulting in an obstacle factor to reduction in size of the ring resonance sensor.